The greater of what your parcel weighs and what its size says it should weigh. Couriers bill on that figure, not on the scale reading.
So a heavy small box is charged by weight, and a big light box is charged by size.
How the size figure is worked out
Length times width times height in centimetres, divided by 5000. That gives a weight in kilograms.
A 40 × 30 × 25 cm box: 40 × 30 × 25 = 30,000, divided by 5000 = 6 kg. If the thing inside weighs 1.5 kg, you are billed for 6.
And for several items
Metzee works out the chargeable weight of each item, adds them up, and rounds the total up to the nearest 10 grams.
Which is why combining two items into one parcel does not always halve the cost: two bulky things are still bulky together.
Why couriers do this
Because a van fills up by volume long before it reaches its weight limit. A courier charging only by weight would be paid almost nothing to carry a van full of cushions.
It is not a surcharge and it is not Metzee's rule. Every courier in the world works this way, under one name or another — volumetric weight, dimensional weight, cubed weight.
What it means for you, practically
Measure the packed parcel, not the item. A 20 cm vase in a 45 cm box of padding is a 45 cm parcel.
Pack tight. Dropping one dimension from 40 cm to 30 cm on that example takes the chargeable weight from 6 kg to 4.5 kg. That is the cheapest saving available to a seller who posts bulky things.
Enter the real figures. If a parcel turns out bigger or heavier than you declared, the courier charges the difference afterwards and it lands on your balance as a later deduction. Guessing low does not save money; it defers it.
The thing most likely to catch you
Anything light and large: lampshades, cushions, boxed toys, framed prints, shoes in their original box. The scale says 600 grams and the courier says 5 kg, and both are right.
If you sell those, price delivery from a measured box before you list, not after you sell. Why is a light thing costing so much to post?
More: why does a big light box cost so much to send, how delivery is charged, how to set delivery prices.